1. Field of the Invention
This invention relates to a color cathode-ray tube, and more particularly to a color cathode-ray tube with a shadow mask unit comprising a rectangular mask frame and a shadow mask fixed to the mask frame.
2. Description of the Related Art
The color cathode-ray tube is generally provided with an envelope containing a face panel and a funnel, a phosphor screen is formed on an inside face of the envelope and the funnel being attached to the face panel. The phosphor screen is composed of a plurality of phosphor layers that have characteristics of red, green, or blue light. In the neck portion of the funnel is arranged an electron gun which emits three electron beams corresponding to luminous colors. Furthermore, inside the face panel between the electron gun and the phosphor screen, a shadow mask unit is provided which has the function of selecting the electron beams emitted from the electron gun by color and causing the selected beam to strike a phosphor layer of a particular luminous color. The unit is supported by the envelope via holders.
The shadow mask unit includes a shadow mask formed of a substantially rectangular thin plate and a rectangular mask frame to which the peripheral portion of the shadow mask is attached. A number of electron beam apertures through which electron beams pass are made in the shadow mask.
In recent years, there has been an increasing demand that the color cathode-ray tube should have a more flat screen to improve visibility. Furthermore, there has been a strong demand that color cathode-ray tubes used for computer displays and the like should have a higher resolution.
To increase the resolution of the color cathode-ray tube, it is necessary to achieve high minuteness by making the aperture pitch of the shadow mask smaller. Since apertures are made in the shadow mask by etching, it is preferable that a plate of a mask material should be thinner to achieve high minuteness. With respect to the mask's mechanical strength, however, it is desirable for the plate to be thicker. Thus, there have been conflicting demands for the plate thickness of a mask material.
Furthermore, the shadow mask is used to select one of the electron beams emitted from the electron gun so as to project the selected one properly on the phosphor layer of the phosphor screen. To accomplish a high quality of the image appearing on the phosphor screen by electron beam scanning, the shadow mask must be arranged at a predetermined position so as to align with particular positions on the phosphor layer.
The amount of electron beams that pass through the electron beam apertures in the shadow mask and reach the phosphor screen is on the order of 20 to 30% of the total amount of electron beams emitted from the electron gun. The remaining greater part of the electron beams collide with the shadow mask, heating the mask and thermally expanding it. The thermal expansion of the shadow mask is noticeable when a very bright image is displayed all over the screen, or when a very bright image is displayed locally on the screen. The thermal expansion causes the shadow mask to deviate from the phosphor layer, degrading the color purity.
To improve the mechanical strength of the shadow mask and prevent the degradation of the color purity, there is provided a installing method wherein a relatively thin, flat shadow mask is attached to a mask frame while tension is applied to the shadow mask. To be precise, one method is welding a pair of long-side edge portions extending horizontally of the shadow mask to the mask frame, with tension applied to the shadow mask in the vertical direction. Another method is welding both of the long and short side edges of the shadow mask to the mask frame with tension applied to the shadow mask in the horizontal and vertical directions.
However, when the shadow mask is secured to the mask frame with tension applied as described above, there arises a problem: since in a rectangular mask frame, the stiffness of the corner portions of the frame is significantly higher than that of the central portion of each frame sidewall, for example, when in the manufacturing process for cathode-ray tubes, the shadow mask unit is cooled after having passed through a high-temperature furnace and been heated, wrinkles will appear near the corners of the shadow mask.
The mechanism of wrinkles appearing in the shadow mask will be explained. When the shadow mask unit is at high temperatures, both of the shadow mask and the mask frame have expanded thermally. When the unit is cooled, the shadow mask whose thermal capacity is small gets cold earlier and shrink. Such a process causes the mask frame to be pulled by the shadow mask and to become distorted. At this time, the amount of bending of the mask frame is smaller near the frame corners whose stiffness is high, with the result that the corner portions of the shadow mask secured to the frame corner portions are stretched, resulting in plastic deformation. Thereafter, when both of the shadow mask and slack if formed in the mask frame get cold, slack if formed in the portions where plastic deformation has occurred in the shadow mask causing wrinkles in the shadow mask.
Consequently, the alignment of the corner portions of the shadow mask with respect to the phosphor layer is lost, degrading the color purity.